The present invention relates to a gasket tape produced from a uniaxially stretched tape mainly comprising polytetrafluoroethylene (PTFE) and a production method thereof.
A PTFE gasket tape is used for various seals, particularly for sealing of chemical equipment and plant in which chemical solutions are handled.
It is known that there are PTFE gasket tapes produced by using a uniaxially stretched tape (U.S. Pat. No. 3,953,566 and U.S. Pat. No. 5,098,625) and a biaxially stretched tape (U.S. Pat. No. 5,964,465 and U.S. Pat. No. 6,080,472).
In case of the uniaxially stretched gasket tape, a high strength is exhibited in the stretching direction but a strength in the direction crossing at right angles to the stretching direction is not sufficient and creep resistance is low. Thus the uniaxially stretched gasket tape is inferior in performance as a sealing material and its use is limited.
On the other hand the biaxially stretched gasket tape is excellent in sealing properties such as a strength and creep resistance in the longitudinal direction and transverse direction (biaxially stretching directions) of the tape. However in order to obtain an article uniformly stretched in the two directions, a high grade quality control is required and as a result, production cost is increased which results in an expensive product.
Also the above-mentioned U.S. Pat. No. 5,964,465 further describes a technique for producing a tape by producing a biaxially stretched film having a thickness as thin as 25 to 510 xcexcm, overlapping a plurality of those thin films (tens of films) to make a sheet and then slitting the sheet. However in order to produce such a thin biaxially stretched film, intensive management is required for production steps, and it is difficult to increase a yield.
An object of the present invention is to provide a method of producing, by a uniaxially stretching method which is easy in production management, a gasket tape which is equal to a gasket tape produced by a biaxially stretching method, and to provide a gasket tape which is produced by the uniaxially stretching method and is excellent in sealing property.
The present invention relates to a method of producing a polytetrafluoroethylene (PTFE) sheet or film, wherein the following steps (1) to (5) are carried out in that order:
(1) a step for preparing a paste comprising a mixture of a polytetrafluoroethylene powder obtained by emulsion polymerization and an extrusion aid,
(2) a step for extruding the paste by means of an extruder equipped with a die having an orifice which has a rectangular cross section and is provided with plural separators inside thereof,
(3) a step for rolling, in the extruding direction, an extrudate having slit lines to give a rolled PTFE sheet,
(4) a step for uniaxially stretching the rolled sheet in the rolling direction after removing the extrusion aid from the rolled sheet, and
(5) a step for heat-treating the uniaxially stretched PTFE sheet at a temperature within a range of 340xc2x0 to 380xc2x0 C.
The above-mentioned die of the extruder is provided with a plurality of sheet-like separators, preferably 2 to 8 separators in parallel with a long side of the orifice having a rectangular cross section. The plural separators are mounted so that any of free ends of the separators is located at a point exceeding a center point of the long side of the orifice. The separators function to slit the inside of the extrudate in parallel with the long side of the orifice.
The gasket tape of the present invention may be produced by immediately subjecting the uniaxially stretched PTFE sheet obtained in the step (4) to heat-treating of the step (5). However a strength of the gasket tape can be increased more by overlapping a plurality of the uniaxially stretched PTFE films obtained in the step (4) and then carrying out the heat-treating of the step (5) or by overlapping a plurality of the uniaxially stretched PTFE films obtained in the step (4) and a plurality of biaxially stretched PTFE films each other preferably alternately and then carrying out the heat-treating of the step (5).
It is preferable that the heat-treating step (5) is carried out at 340xc2x0 to 380xc2x0 C. for 30 minutes or more.
In case of producing products having a given width such as a gasket tape, slitting into a given width may be carried out after the heat-treating step (5) or the heat-treating step (5) may be carried out after the slitting into a given width is carried out subsequently to the uniaxially stretching step (4).
Also a gasket tape having an adhesive layer may be produced by carrying out a step (6) for providing an adhesive layer on at least one surface of the tape after the heat-treating step (5).
The present invention also relates to a PTFE gasket tape which is obtained by heat-treating the uniaxially stretched PTFE sheet and has a matrix tensile strength (MTS) of 2,300 to 5,700 psi in the longitudinal direction (MTSL) and 130 to 660 psi in the transverse direction (MTST) and MTSL/MTST of about 8 to about 18, or a PTFE gasket tape which is obtained by heat-treating a plurality of uniaxially stretched PTFE films overlapped each other or heat-treating uniaxially stretched PTFE films and biaxially stretched PTFE films overlapped each other preferably alternately and has a matrix tensile strength (MTS) of 2,500 to 8,000 psi in the longitudinal direction (MTSL) and 200 to 700 psi in the transverse direction (MTST) and MTSL/MTST of about 8 to about 11.